1. Field of the Invention
The present invention relates to a liquid recovery method and system for a compressor that encases inside a housing a compression mechanism and a reservoir for holding liquid used to lubricate sliding parts including the compression mechanism.
2. Description of the Related Art
The compressor of this kind is made air-tight by connecting a housing to a refrigeration cycle. Driving the compression mechanism introduces refrigerant from the cycle into the housing from a suction port, compresses and discharges the refrigerant into the space inside the housing, and returns the refrigerant back to the cycle from a discharge port. Lubricating oil in a reservoir inside the housing is supplied to sliding parts including the compression mechanism either alone or mixed in the refrigerant to provide lubrication. The compressor is thus basically maintenance-free. Generally, the refrigerant discharged from the compression mechanism back to the refrigeration cycle contains lubricating oil, which can be the cause of various problems in the cycle. A compressor with a lubrication system in which lubricating oil flows out into the refrigeration cycle in a large amount tends to be bulky because it necessitates a large oil reservoir to prevent insufficient lubrication of sliding parts.
Some known lubricant recovery systems use techniques (so called “cyclone system”) of centrifugally separating lubricating oil from to-be-discharged refrigerant before it flows out into the refrigeration cycle, so as to return the oil to the reservoir inside the housing (for example, see Japanese Patent Laid-Open Publication Nos. Hei 7-151083 (patent document 1), Hei 11-82352 (patent document 2), 2001-20865 (patent document 3), 2001-280552 (patent document 4), 2002-115686 (patent document 5)). Typically, a cylindrical centrifugal separator is arranged orthogonally to the axial line of the compressor, into which refrigerant discharged from the compression mechanism is introduced from an upper part along a tangential line, so as to create a downward whirl along the cylindrical surface. Refrigerant flows upward from the bottom of the separator through a central portion and returns to the refrigeration cycle, while the lubricating oil that is separated from the refrigerant under centrifugal force is drained from a lower part of the separator to return to the reservoir in the housing.
Patent document 2 shows a system in which centrifugally separated lubricating oil is blown out parallel to the oil surface of the reservoir so as not to disturb the oil surface, whereby the oil level in the reservoir is maintained constant and the supply of oil to the sliding parts made stable, and also, any backflow of lubricating oil in the reservoir because of the disturbed oil surface is prevented.
Patent documents 3 to 5 show compressors in which an annular dividing wall is provided between the compression mechanism and the stator of the electric motor that drives the mechanism. More particularly, the dividing wall disclosed in the patent document 3 includes partitions that divide the annular space between the dividing wall and housing into left and right chambers for flowing-in refrigerant and flowing-out refrigerant, respectively. This structure creates a smooth flow of refrigerant discharged from the compression mechanism, from the flowing-in refrigerant chamber into the housing through the electric motor towards the reservoir in the lower part, where it turns upward and flows out of the housing through the flowing-out refrigerant chamber. The in-coming refrigerant hits against the stator of the electric motor and makes a sharp turn, whereby lubricating oil is separated efficiently by centrifugal force.
Patent documents 4 and 5 show structures in which refrigerant discharged from the compression mechanism is led toward inside of the dividing wall. The refrigerant is then guided downward in a swirl through the rotor of the electric motor together with lubricating oil used for lubricating the bearing, which forms another sliding parts in the compression mechanism around the drive shaft. The lubricating oil is centrifugally separated from refrigerant as the refrigerant hits against the inner face of the stator or when it travels downward from the bottom of the rotor, so that refrigerant is discharged to the outside of the housing after lubricating oil has been removed.
Meanwhile, demands for lighter vehicles equipped with such air conditioner compressors are increasing to address growing energy and environmental issues. The down-weighting is particularly crucial for electric vehicles or gasoline-electric hybrid vehicles, whose drive power obtained from batteries is not as high as that of gasoline vehicles. The vehicle-mounted electric compressor that includes a large and heavy electric motor should obviously be as small and light as possible.
The inventors of the present application have endeavored to achieve downsizing and down-weighting of compressor by downsizing of compression mechanism and electric motor inside the housing and by reduction of dead space inside the housing, and in these research and development the inventors have encountered the problem of insufficient lubrication in sliding parts such as the compression mechanism.
Through various tests and investigations the inventors found that the insufficient lubrication was caused by a delay in the replenishment of lubricating oil: While the reservoir holds less amount of oil because of the downsizing of the housing, recovery of the oil that has been supplied to sliding parts is not made quickly enough, even with the oil separation systems shown in the above-mentioned patent documents 3 to 5.
Other systems in the art include those shown in the patent documents 1 and 2, in which oil that has lubricated sliding parts is all discharged with refrigerant through the compression mechanism and is separated and recovered. With such systems, the problem of insufficient lubrication will not occur if oil separation is made efficiently. To ensure sufficient oil separation, however, the system needs to be complex and bulky, contrary to the demands for downsizing and down-weighting. Further, this system does not provide lubrication to sliding parts other than the compression mechanism.
In the system shown in the patent document 3, the refrigerant that flows into the housing from the compression mechanism toward the electric motor does not contact lubricating oil that is discharged after lubricating the bearing, which forms sliding parts other than the compression mechanism around the drive shaft, because of the dividing wall. Any oil entrained in the refrigerant is separated and returned to the reservoir. Although the oil is not collected immediately after the refrigerant is discharged, the amount of oil carried in the refrigerant is small and it is efficiently recovered. However, the amount of lubricating oil that is used for lubricating the bearing is much larger than the oil entrained in the refrigerant, and it takes time to recover this oil; because it drops onto the rotor of the electric motor and is scattered around by the rotor or balance weight, it travels downwards only when it is urged onto the dividing wall or inner face of the stator by centrifugal force, and is collected in the reservoir when it drops from the bottom of the stator. Thus this portion of lubricating oil can cause insufficient lubrication.
In the systems shown in the patent documents 4 and 5, the refrigerant that flows into the housing from the compression mechanism toward the electric motor is led inside of the dividing wall and blown into the inside of the stator, towards the reservoir. The refrigerant carries with it the lubricating oil that has lubricated the bearing, which is centrifugally separated by the rotation of the rotor or balance weight. Oil is thus collected somewhat quicker than the system shown in the patent document 3 but still has to travel long. Further, an attempt to increase the recovery speed will cause more oil to pass through the rotor without being separated from the refrigerant, resulting in a lower recovery rate. Thus there is still scope of improvement in respect of prevention of insufficient lubrication.
Furthermore, none of the oil separation systems shown in the patent documents 3 to 5 is applicable to horizontal compressors.